Absorption spectroscopy is a well known analytical technique used to determined the concentration of one or more substances in a sample. In most cases, absorption spectroscopy is performed in a device such as spectrophotometer that comprises in its most basic form, a light source, a sample holder and a detector. Light produced from the light source travels through the sample holder to a detector which measures the amount of light reaching it. In the absence of a sample, light from the source passes unobstructed to the detector. In the presence of a sample, light produced by the light source must pass through the sample before reaching the detector. Prior to interacting with a sample, light produced from a source is usually termed incident light, whereas light that is transmitted through the sample to the detector is called transmitted light. Incident light and transmitted light may differ for example in intensity, wavelength or both. When incident light strikes a sample composition, a portion of the incident light may be reflected, scattered or absorbed by components of the sample. These changes account for the differences between the incident light and transmitted light.
For a non-scattering sample, absorbance of the sample is proportional to the log of amount of incident light illuminating a sample divided by the amount of light transmitted through the sample. The incident light can be measured in a single-beam spectrophotometer by measuring the amount of light reaching the detector with no sample in the chamber. When the sample chamber does not contain a sample, the incident light is often referred to as the reference beam. The incident light for a non-scattering sample is usually less than the amount of light required to saturate the detector, i.e., when the detector has achieved its maximum linear response to changes in light intensity. In order for light to be transmitted through a highly light scattering sample, the intensity of the incident light must be significantly higher than the amount of light required to saturate the detector. In such a case, it is difficult to determine the true intensity of the incident light.
One method of compensating for detector saturation is to use a smaller integration time for the reference measurement, that is the time that the detector remains exposed to the light before the actual signal is collected. However, the use of different integration times for the sample measurements and the reference measurements can lead to error in analyte determinations. Further, when different integration times are used for sample measurements, and reference measurements, the log of the ratio of the sample integration time to reference integration time, must be added to the absorbance value. In such a case, the absorbance of a light-scattering sample may not necessarily be indicative of the true absorbance of the sample.
A second method of compensating for detector saturation is to attenuate the reference beam with a filter or photometric reference member. A photometric reference member or filter may be used to reduce the intensity of the incident light reaching a detector. The photometric reference member may comprise a light-scattering substance, or a neutral density filter. In cases where temperature variations may exist between samples and the analyte being assayed is very low, the absorbance of the photometric reference member should be resistant to temperature changes and the noise level of the spectrophotometer should be very low. For example, for the non-invasive measurement of glucose in a finger, any fluctuation in the contribution of absorbance from the photometric reference member as a result of temperature fluctuations, could affect the accuracy of predicted glucose concentrations.
WO 01/15596 discloses an artificial member which mimics the absorbance spectrum of a body part and includes the spectral components of blood. The application discloses an artificial member that comprises a material which provides a scattering effect similar to tissue such as the skin or a digit, for example, Teflon-PTFE with 25% glass fibers. However, a drawback to the artificial member described in the application is that material comprising 75% PTFE and 25% glass fiber does not exhibit absorbance-related temperature resistance over wavelength ranges that are often used in analytical spectroscopy procedures.
U.S. Pat. Nos. 6,015,610 and 5,596,450 disclose a thin, moldable, flexible and highly reflective material to provide diffuse light. The patents teach the use of expanded polytetrafluoroethylene (PTFE) comprising a microstructure of polymeric nodes and fibrils. A drawback of the materials disclosed in these patents is that PTFE by itself does not exhibit absorbance-related temperature resistance over wavelength ranges used in many analytical spectroscopy procedures.
There is a need in the art for a photometric reference member whose absorbance is resistant to temperature changes. Further, there is a need in the art for a photometric reference member whose absorbance is resistant to temperature changes and that is highly light scattering so that any slight change in the positioning of the photometric reference member within an analytical device does not substantially affect the amount of diffuse light transmitted to the detector.
It is an object of the present invention to overcome disadvantages of the prior art.
The above object is met by a combination of the features of the main claims. The sub claims disclose further advantageous embodiments of the invention.